When a vehicle makes a turn, a tread portion of a tire that has a slip angle slips partially against the road surface. In other words, in a contact region of the tread portion of the tire, there mixedly exist a region (adhesion region) in which the tread portion adheres to the road surface and a region (slip region) in which the tread portion slips against the road surface. As the slip angle increases, the adhesion region is decreased while the slip region is increased, which finally leads to the whole contact region of the tire becoming the slip region, and the tire as a whole causing slide slipping (slide slip behavior).
Generally, the slide slip behavior is difficult for the vehicle driver to detect, and moreover, once the slide slip behavior occurs, suppressing the slide slip behavior to restore the steering under control is almost impossible. Consequently, the vehicle experiences out of steering control which in turn could result in a vehicle crash.
In an effort to solve such problems, there have been control systems of anti-lock braking systems (ABS) that are mounted on vehicles to prevent occurrence of slide slipping. For instance, in ABS systems, a slip ratio of the tire is estimated by using measured rotation speeds and traveling speeds of the tire, and the occurrence of the slide slip behavior is estimated based on measured data of rotation torque applied to the vehicle axle, so that the brake is controlled to prevent the possibility of occurrence of the slide slip behavior.
However, since such control is achieved by measuring the forces and speeds that are applied to the tire as a whole, it has not always been able to estimate the occurrence of the slide slip behavior with accuracy. Particularly, it has been unable to estimate the slide slip behavior based on information of the slip region in the contact region of the tire.
The following Patent Documents 1 to 3 disclose, for instance, a method in which an acceleration sensor is attached to a tire to acquire measurement data of acceleration of the tire during rotation, and from the acquired measurement data, power spectrums and vibration spectrums are obtained to estimate the status of the road during the tire rotation, and a method in which a timing at which a tread portion is in contact with a road surface is determined from measurement data of acceleration in a radial direction.
However, any one of the Patent Documents 1 to 3 does not allow detection of a slide slip state from the measurement data of acceleration, though it is possible to estimate the status of the road surface using the measurement data.
Patent Document 1: JP 2002-340863 A
Patent Document 2: JP 2003-182476 A
Patent Document 3: JP 2002-511812 A